I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc. But I don't know why. Could I persuade anyone to enlighten me, please?

While there's a lot of optical theory involved in an exact answer, most of which is over my head (things such as circle of confusion, degree of enlargement, etc), the primary reason is relatively simple.

To get the equivalent field of view and image framing with a larger format camera, you must use a longer lens at the same focus distance. Think of it this way, if you're shooting 35mm with a 200mm lens at 8 feet distance, your depth of field is very shallow, even at f/8. Now a large format shooter has to do the same thing (200mm lens, 8 feet) just to shoot a waist up portrait. They're still stuck with the depth of field of a 200mm lens, focused at 8 feet and f/8, even though the field of view is closer to a 55mm lens (in 35mm format).

DOF is proportional to magnification. Not just at the sensor, but also in relation to display size. When looking at an 8x10 print, which image was enlarged more, an APS size one, or a full-frame 35mm one? So when using the same focal length, the APS image is being enlarged more to reach the same display size and suffers a slight loss of DOF - notice, the angle of view is not the same. (If the display sizes are changed in relation to the different sensor sizes then you are back to the same DOF.)

However, when at the same angle of view, which means different focal lengths and image plane magnifications, the smaller sensor has greater depth of field.

Anon E Mouse is right that the issue of degree of enlargement affects depth of field. It is a little confusing in that the principle works in what you would think is the wrong direction. For example, small formats have to be enlarged more to reach a given print size, so they in theory would have less depth of field (more enlargement = less depth of field).

However, having to use a longer focal length lens to achieve an equivalent angular field of view has a much larger effect, so the net result is that larger formats have functionally less depth of field at comparable angular fields of view, focus distance, and aperture.

For a sense of the scale of the issue, here are some rough guidelines to think about in terms of depth of field.

A 1.6 crop camera has roughly 1 1/3 stop more depth of field than a full frame camera. (ie. f/1.8 on 20D with 50mm lens = ~f/2.8 on 5D with 80mm lens).

A 35mm full frame camera has about 4 1/2 stops more depth of field than a 4x5 camera. (ie. f/1.4 on 5D with 35mm lens = ~f/6.4 on 4x5 with 135mm lens)

This conversation suggests that a less-than full frame sensor increases subject magnification and affects DOF. It does not.

It is a misnomer to refer to the effect of a smaller sensor as a multiplier, a multiplication effect, etc. Camera manufacturers label it that way, and that is not at all what happens. It confuses people.

A 100mm lens still has a 100mm focal length, whether it is used on a full frame sensor or not. The sensor size merely affects the field of view, not the subject magnification.

I have used a Canon 180mm "L" macro lens for years. Whether I use it on my 1Ds MkII or my Canon 20D, I get exactly the same size subject if I use a tripod and keep the subject distance fixed. Since the subject distance and subject magnification are unaffected, so is the DOF. All of this is very easy to demonstrate.

What will be different is that the 1Ds MkII image will show a wider FOV. The FOV when the same lens is used on a 20D is much narrower. It is approximately the same as if the lens was 270mm. This is often a benefit for macro shots because it makes it easier to separate the subject from the background.

When you use a smaller sensor but keep the focal length the same with a 35mm lens, the smaller sensor captures a smaller part of the image from the lens. The lens is still designed (and used by the camera) to create an image the size required for 35mm film. You just get the central portion of that image with a smaller sensor. The rest falls outside the sensor. The portion you do get is identical to what you get with a full-frame sensor.

I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc. But I don't know why. Could I persuade anyone to enlighten me, please?[a href=\"index.php?act=findpost&pid=65745\"][{POST_SNAPBACK}][/a]

If we try to compare depth of field performance on two different cameras, we need to adopt some premises for comparison.

Here is one set that is often used:

1. We use focal lengths on the two cameras that yield the same field of view.

2. We have the subject at the same distance.

3. We use an apertureof the same f/number.

4. We use, as our criterion of acceptable blurring, circle of confusion diameter limit (COCDL) values that are the same fraction of the camera's format size.

Note that condition 4 is consistent with saying that we will judge blurring on the same size print, viewed from the same distance, for both cameras.

If we adopt those premises for comparison, then the camera with the larger format will have less depth of field.

As to why this is, it is easiest to say "it comes form the depth of field equations". The parameters that are different between the two situations are:

A. The focal length is greater on the larger-format camera (per item 1). That greatly reduces the depth of field (it goes roughly as the inverse of the square of the focal length). The reason has to do with longitudinal magnfication and the relationship between depth of focus and depth of field, but I won't bore you with that right now.

B. Our tolerance for the size of the blur figure is increased (per item 4). That of course leads to an increased depth of field, but not to the degree required to overcome item A (it goes roughly as the COCDL).

If we try to compare depth of field performance on two different cameras, we need to adopt some premises for comparison.

Here is one set that is often used:

1. We use focal lengths on the two cameras that yield the same field of view.

2. We have the subject at the same distance.

3. We use an apertureof the same f/number.

4. We use, as our criterion of acceptable blurring, circle of confusion diameter limit (COCDL) values that are the same fraction of the camera's format size.

Note that condition 4 is consistent with saying that we will judge blurring on the same size print, viewed from the same distance, for both cameras.

If we adopt those premises for comparison, then the camera with the larger format will have less depth of field.

As to why this is, it is easiest to say "it comes form the depth of field equations". The parameters that are different between the two situations are:

A. The focal length is greater on the larger-format camera (per item 1). That greatly reduces the depth of field (it goes roughly as the inverse of the square of the focal length). The reason has to do with longitudinal magnfication and the relationship between depth of focus and depth of field, but I won't bore you with that right now.

B. Our tolerance for the size of the blur figure is increased (per item 4). That of course leads to an increased depth of field, but not to the degree required to overcome item A (it goes roughly as the COCDL).[a href=\"index.php?act=findpost&pid=66759\"][{POST_SNAPBACK}][/a]

Let's not confuse our discussion with unnecesary talk about the circle of confusion.

The circle of confusion is the same when both cameras are 35mm DSLRs. Their sensors are positioned the same distance from the lens. The lens has the same focal length, etc.

With a less than full-size sensor, you are getting a smaller central portion of that same circle of confusion. That's all.

Imagine that you took 35mm film and painted an opaque back ink frame around the film so that only the central 2/3 of the film frame is exposed to the light during exposure. Do you believe you get subject magnification or affect DOF? Well, you wouldn't. Ask anyone who's had the film transport in a 35mm film camera go screwy.

A less than full-size sensor has no sensors around the periphery of a 35mm frame, just as if you blocked the extra sensors of a full-frame sensor with something opaque. That's the only difference in a 35mm DSLR. All you get is reduced FOV. You do not get a boost in magnification, subject or otherwise.

You cannot separate the issue of depth of field from the concept of a circle of confusion. Depth of field is a construct that is defined by the application of a chosen circle of confusion. You change the arbitrary number that is the circle of confusion, and you change the depth of field. Talking about one without the other is like trying to write in English without using the alphabet.

Fortunately, we have standarized on a circle of confusion that represents the average person's ability to see detail in an 8x10 print at normal viewing distances. Change the print size, viewing distance, or the degree to which you have to enlarge your "negative" to get to an 8x10 print, and you have to change the circle of confusion, which changes the depth of field. Of course, it changes no true optical properties of the lens/camera/film, it just changes what we perceive to be in focus.

You are correct that sensor/negative size has nothing to do with "magnification" as though a smaller sensor was a magic teleconverter. However, it does have everything to do with enlargement. The more you have to enlarge a negative to get to a standard print size, the more stringent a circle of confusion you have to use on the negative to have the standard print be acceptably sharp (acceptable DOF). In the context of this parameter, smaller formats actually have less depth of field, because they must be enlarged more.

However, this is overwhelmingly outweighed by the key issue (which is the answer to the original question). When you use a smaller format, you tend to use a shorter focal length lens and stand farther away with that lens than you would with a larger format.

You are correct that sensor/negative size has nothing to do with "magnification" as though a smaller sensor was a magic teleconverter. However, it does have everything to do with enlargement. The more you have to enlarge a negative to get to a standard print size, the more stringent a circle of confusion you have to use on the negative to have the standard print be acceptably sharp (acceptable DOF). In the context of this parameter, smaller formats actually have less depth of field, because they must be enlarged more.

However, this is overwhelmingly outweighed by the key issue (which is the answer to the original question). When you use a smaller format, you tend to use a shorter focal length lens and stand farther away with that lens than you would with a larger format. [{POST_SNAPBACK}][/a]

That's adding other factors besides the size of the sensor. That shifts the argument from a discussion of sensor size and DOF to sensor size, DOF, and other stuff.

The discussion was about whether an APS-C size sensor in a 35mm DSLR alters DOF or subject magnification. We both agree, it does not.

Alter anything else, of course things will change. But that misses the "key" issue in these DOF threads.

If you alter the distance to change the FOV, of course you will alter the DOF. But that has nothing to do with the sensor size. It has everything to do with repositioning the camera. Why confuse the issue?

You cannot get the same subject magnification and FOV with a full-frame sensor and a smaller sensor at the same subject distance. If you want the same subject size, you have to sacrifice FOV for the smaller sensor. Want the same FOV, you have to sacrifice subject magnification.

Same lens and same subject distance, full-frame or not, you get the same reproduction ratio. Use the same enlargement for both images, and nothing changes. Use different enlargements for your images and any differences in image appearance owe to those different enlargement ratios, not the differences in the size of the sensors.

I believe it is helpful, in discussions like this, to focus on what changes and what does not. If we hold everything else constant and change only the sensor size, the only thing that changes is FOV. Subject magnification and DOF are completely unaffected.

I do not tend to stand any further back with my 20D than I do with my 1Ds MkII. The relationship between subject and background and the overall effect I want the image to have on the viewer determine what focal length lens I use and where I stand.

Many people assume they get less DOF with a smaller sensor because of a telephoto multiplier effect. It is that widely held misunderstanding that I have been addressing.

The conversation about Circle of Confusion just adds to the confusion. That is a factor determined by the lens manufacturer for the expected enlargement factor. Medium format film cameras use a lower Circle of Confusion because the expected enlargement ratio is smaller. Same lens on two 35mm DSLRs with the same subject distance and aperture, one full-frame and one not, yields the same Circle of Confusion and the same DOF. You just see less of it with the samller sensor.

I see this argument all over the different forums. With a 10D or a 20D or whatever, you get a 1.6x multiplier effect, so your 300mm lens becomes the equivalent of a 480mm lens. (It does, but in FOV only.) The logic is then extended to DOF. Since the lens is the equivalent of a 480mm lens, DOF must be the same as a 480mm lens. Nope. For subject magnification and DOF, the lens is still the equivalent of 300mm.

Michael Reichmann has a really nice essay on DOF, Circle of confusion, etc.:

The only issue is that to measure DOF, you must choose a circle of confusion. In reality there is no DOF, only focus plane then gradually increasing blur. DOF simply is a measurement of how much blur we will tolerate before something becomes "out of focus". You have to draw the line between "acceptable focus" and "out of focus" somewhere, which is done by picking a circle of confusion. You can pick one, I can pick one, the lens manufacturer can pick one, but it's still an arbitrarily chosen number that's the determining factor of how much "DOF" you have.

I guess I should also clarify my statement on the "standing further back" point. You only stand further back with a smaller format if you hold the lens focal length constant. I should have said that with a smaller format you use a shorter focal length lens OR you stand further back.

Another good article on DOF, more relating to the issue of full frame 35mm vs. 1.6 crop digital:

The only issue is that to measure DOF, you must choose a circle of confusion. In reality there is no DOF, only focus plane then gradually increasing blur. DOF simply is a measurement of how much blur we will tolerate before something becomes "out of focus". You have to draw the line between "acceptable focus" and "out of focus" somewhere, which is done by picking a circle of confusion. You can pick one, I can pick one, the lens manufacturer can pick one, but it's still an arbitrarily chosen number that's the determining factor of how much "DOF" you have. [a href=\"index.php?act=findpost&pid=66821\"][{POST_SNAPBACK}][/a]

To determine the width of the DOF, yes, I agree. But that has nothing whatsoever to do with DOF and sensor size.

That's why I say, discussion of Circle of Confusion only adds confusion here. It is a constant when we talk about full-frame sensor v. smaller sensor. It's a relevant factor in other contexts. But not when we compare the results we can expect with a full-frame sensor or a smaller sensor using the same lens.

Let's not confuse our discussion with unnecesary talk about the circle of confusion. [a href=\"index.php?act=findpost&pid=66803\"][{POST_SNAPBACK}][/a]

I was not talking about some circle of confusion, or even the diameter of some circle of confusion. I was speaking of the circle of confusion diameter limit (COCDL) which we adopt as a criterion of acceptable blurring, without which concept there is no such thing as depth of field. (The ambiguity in terminlogy is why I don't like calling this adopted number "the circle of confusion", and I don't call it that.)

Quote

The circle of confusion is the same when both cameras are 35mm DSLRs. Their sensors are positioned the same distance from the lens. The lens has the same focal length, etc.

Would a 35-mm dSLR be one whose sensor size is 36 mm x 24 mm?

Indeed, if we have a camera with a certain focal length lens, focused at a certain distance, with a certain aperture (as an f/number), then for a point on an object at some specific distance other than the focus distance, the diameter of the circle of confusion (its actual diameter, not a limit we place on that diameter as a criterion of "acceptable blurring") will be the same regardless of format size. (The optical system of course "has no idea" how large is the film mask or digital sensor array at the focal plane.)

But maybe that's not what you mean.

Maybe you mean, "for a given format size, many people use the same circle of confusion diameter limit for reckoning depth of field." But hardly always.

Quote

With a less than full-size sensor, you are getting a smaller central portion of that same circle of confusion.

I can't imagine what you mean by that. Are you by any chance confusing "circle of confusion" with "image circle"?

Regarding magnification, (image) magnfication is a function of the focal length and the distance to the subject. I certainly didn't say anything about (image) magnfication. I discussed longitudinal magnfication, which is a different matter althogether. It is involved in relating depth of focus to the corresponding depth of field.

In any case, I'm afraid I'm not able to follow the point you are making.

1) DoF is a function of aperture, focal length and subject distance from the camera, or more simply stated: aperture and subject magnification factor. Note that sensor size has nothing to do with either equation.

2) However, to obtain a similar FRAMING from cameras with different sensor sizes but using the same focal length lens, we must change our shooting position -- which in turn changes the magnification factor, which in turn changes the DoF. So it is ALWAYS true that smaller sensor cameras will have MORE DoF when generating framing comparable to a larger-sensor camera, all else equal.

3) CoC diameter is a CHOICE and hence is a variable in the DoF equation. As a result it always generates some confusion when discussed. Simply stated, for a given PRINT size and resolution desired (photographer's choice and hence variables), CoC will need to change based on sensor size; smaller sensors will require a smaller CoC to generate the same print resolution as the larger sensor, but the change is not proportional.

3a) IMO the confusion arises because the CoC required does NOT change proprtionately with subject magnification factor, it changes as the inverse square -- which again is why a smaller sensor will generate MORE DoF in a given image when all else is equal.

1) DoF is a function of aperture, focal length and subject distance from the camera, or more simply stated: aperture and subject magnification factor. Note that sensor size has nothing to do with either equation.

2) However, to obtain a similar FRAMING from cameras with different sensor sizes but using the same focal length lens, we must change our shooting position -- which in turn changes the magnification factor, which in turn changes the DoF. So it is ALWAYS true that smaller sensor cameras will have MORE DoF when generating framing comparable to a larger-sensor camera, all else equal.

3) CoC diameter is a CHOICE and hence is a variable in the DoF equation. As a result it always generates some confusion when discussed. Simply stated, for a given PRINT size and resolution desired (photographer's choice and hence variables), CoC will need to change based on sensor size; smaller sensors will require a smaller CoC to generate the same print resolution as the larger sensor, but the change is not proportional.

3a) IMO the confusion arises because the CoC required does NOT change proprtionately with subject magnification factor, it changes as the inverse square -- which again is why a smaller sensor will generate MORE DoF in a given image when all else is equal.[a href=\"index.php?act=findpost&pid=66850\"][{POST_SNAPBACK}][/a]

Agreed. Except that in #3a you state that the change in the CoC is why the smaller format has more DOF. Actually, the CoC works against the smaller format (opposite direction), but the reasons you stated in #2 completely outweigh that and lead to the smaller format having less DOF.

Except that in #3a you state that the change in the CoC is why the smaller format has more DOF. Actually, the CoC works against the smaller format (opposite direction), but the reasons you stated in #2 completely outweigh that and lead to the smaller format having less DOF.[a href=\"index.php?act=findpost&pid=66855\"][{POST_SNAPBACK}][/a]

You misunderstood -- and I agree the smaller CoC works against the smaller format. As I said, they did not change proportionately but CoC changes at a slower rate, the inverse square. As such, the change required for a smaller CoC with the smaller format progresses less than the subject magnification change does to generate the same framing as the larger-sensor image.

I was speaking of the circle of confusion diameter limit (COCDL) which we adopt as a criterion of acceptable blurring, without which concept there is no such thing as depth of field.

[snip]

Would a 35-mm dSLR be one whose sensor size is 36 mm x 24 mm?

[snip]

[a href=\"index.php?act=findpost&pid=66847\"][{POST_SNAPBACK}][/a]

A 35mm DSL does not need to have a full frame 35mm sensor. It needs to be able to use optics designed for 35mm film cameras. The Canon D30/D60/10D/20D/etc. are all 35mm DSLRs. They use the same Canon EOS lenses used with 35mm film cameras and full-frame 35mm DLRs like the Canon 1Ds MkII.

The optical system in 35mm DSLRs are designed to create images for 35mm film cameras or 35mm DSLRs. As you say, the lens knows nothing about the medium capturing the image.

Some of the 35mm DSLRs cameras just happen to capture only a smaller portion of the image than 35mm film or a full-frame 35mm sensor would capture.

In my opinion, talk about Circle of Confusion, Circle of Confusion Diameter Limit, and the like contribute nothing to a beginning digital photographers understanding about how FOV, DOF, and subject magnification are affected by the APS-C size sensor in a Canon D30/D60/10D/20D/etc. relative to a full-frame sensor.

Camera sales people often tell them that their telephoto lenses get a 1.6x multiplier effect. They tell them that their 300mm lens becomes the equivalent of a 480mm lens. They tell them you get more DOF with the smaller sensor. Etc. At best, the camera sale people misunderstand. At worse, they downright defraud people.

You do not need to know anything about CoC or COCDL to understand that the only thing that changes is FOV when the sensor is smaller.

What you do about that smaller FOV is a separate issue.

How you calculate DOF and the role of CoC is also a separate issue.

The only difference between a larger sensor and a smaller sensor -- everything else being constant (i.e., the same distance to the subject, the same aperture, the same lens, the same focal length) -- is just one thing: FOV.

You can say, most people will reframe. I have no idea what "most" people will do. But when they do reframe -- large sensor or small sensor -- things are gonna change. That's not because of sensor size. That's because you reframed: something completely separate and independent of sensor size.

Sensor size doesn't affect DoF; the Nikon D2X in effect has two sensor sizes, the 12MP APS-C sensor, and in "High-speed crop mode", a smaller 6MP sensor obtained by masking out about half the sensor area. The resulting image, in terms of DoF and everything else, is completely equivalent to doing an identical crop in Photoshop of the full-sensor image. (Nikon's purpose in doing the crop in-camera is to reduce file size to increase the fps rate.) The same is true in comparing a full-frame sensor with an APS-sized one.

I know from hearing snippets of conversations that reduced sized sensors have different DoF characteristics than that of a FF sensor with same lens, f/stop, etc. But I don't know why. Could I persuade anyone to enlighten me, please?[a href=\"index.php?act=findpost&pid=65745\"][{POST_SNAPBACK}][/a]

This has been discussed many times in this forum, and I believe there is a consensus that:

1) If you take photos from the same distance and compare DOF on equal sized prints viewed from an equal distance, the DOF will be the same for any combination of focal length f and f-stop N that gives the same effective aperture diameter, A=f/N. For example, f=50mm, f/1.4, f=70mm f/2 and f=100mm f/2.8 all have effective aperture diameter about A=35mm, and so all give about equal DOF in a "same print size, same distance" comparison.

In other words, adjust f-stop in proportion to focal length to get equal DOF.

2) Smaller formats typically involve using shorter focal lengths to get the same framing of the subject (same FOV), and so will get the same DOF with a proportionately lower f-stop (and thus potentially with a lower ISO speed), or will give more DOF with the same f-stop.

Sometimes the "shorter, brighter" lens needed for equally low DOF does not exist for a smaller format (50/1.4, 85/1.2 or 85/1.4 in 35mm have no matches for for smaller formats). Other times a "match" does exist and then is usually similar in cost and weight, due to having similar sized lens elements, due to the similar aperture diameter (135/2 vs 85/1.4, 200/2.8 vs 135/2, 300/4 vs 200/2.8, 300/2.8 vs 200/2, 600/4 vs 400/2..

Historically, lenses for larger formats have typically had longer focal lengths and higher minimum f-stops, and have been most often used with higher f-stops in order to meet DOF needs, leading to the pattern of larger formats typically being used with lower shutter speeds, or alternatively higher ISO speeds to get the same DOF and shutter speed.